In a portable telephone with a built-in digital camera, an imaging lens is mounted on a printed circuit board. As a method for mounting an imaging lens on a printed circuit board, a reflow soldering processing is used. Hereafter the reflow soldering processing may simply be called “reflow processing”. Reflow processing is a method for soldering an electronic component on a printed circuit board by placing a solder ball in advance at a location where an electronic component is connected, placing the electronic component there, heating to melt the solder ball, then cooling the solder down.
Generally in mass production steps, a reflow step for performing reflow processing is used as a method for mounting electronic elements or such components as an imaging lens on a printed circuit board. If a reflow step is used, the mounting cost of components on a printed circuit board can be decreased, and the manufacturing quality can be maintained at a predetermined level.
In the reflow step of the manufacturing steps of a portable telephone comprising an imaging lens, not only are electronic components arranged at predetermined positions on a printed circuit board, but also the imaging lens itself and a socket for installing the imaging lens are arranged on the printed circuit board.
The imaging lenses installed in portable telephones are largely made of plastic in order to decrease the manufacturing cost and to insure lens performance. Therefore a heat resistant socket component is used for installing the imaging lens, in order to prevent thermal deformation of the imaging lens in a high temperature environment, which makes it impossible to maintain optical performance thereof.
In other words, in the reflow step, a heat resistant socket component for installing an imaging lens is mounted on the printed circuit board of the portable telephone, and the imaging lens is installed in this socket after the reflow step, so that the imaging lens is not exposed to high temperature in the reflow step (see Patent Documents 1 to 3). However using a heat resistant socket component for installing the imaging lens makes the manufacturing steps complicated, and increases the manufacturing cost, including the cost of this heat resistant socket.
Recently it has been demanded that the optical performance of the imaging lens installed in a portable telephone does not deteriorate even if the portable telephone itself is placed in about a 150° C. high temperature environment, considering the case of a portable telephone that is left in an automobile which temporarily becomes a high temperature environment. A conventional imaging lens made of plastic material cannot meet this demand.
In order to implement an imaging lens of which optical performance is maintained even in a high temperature environment, forming an imaging lens using a high softening temperature mold glass material is possible (e.g. see Patent Document 4). The temperature at which a high softening temperature mold glass material softens is several hundred degrees or more, so the deterioration of the optical performance of an imaging lens in a high temperature environment can be avoided, but at the moment an imaging lens made of mold glass material is not popular since the manufacturing cost is very high.
In addition to the above thermal characteristics, an imaging lens installed on a portable telephone must satisfy the following conditions related to optical characteristics. One condition is that the optical length must be short. An optical length is a length from an entrance plane at an object side to an image formation plane (also called the “image sensing plane”) of the imaging lens. In other words, when a lens is designed, the ratio of the optical length to the composite focal lengths of the imaging lens must be minimized. In the case of a portable telephone, for example, this optical length must at least be shorter than the thickness of the portable telephone main unit.
On the other hand, a back focus, which is defined as a distance from the exit plane on the image side face of the imaging lens to the image sensing plane, should be as long as possible. In other words, when the lens is designed, the ratio of the back focus to the focal lengths must be maximized. This is because such components as a filter or cover glass must be inserted between the imaging lens and the image sensing plane.
In addition to the above, imaging lenses are of course required to have various aberrations corrected sufficiently that image distortion is not noticed visually, and moreover aberrations should be corrected so as to be sufficiently small as required by the integrated density of the image sensing element (also called “pixels”). That is, the various aberrations must be satisfactorily corrected; hereafter, an image in which aberrations have been satisfactorily corrected may be called a “satisfactory image”.
Patent Document 1: Japanese Patent Application Laid-Open No. 2006-121079
Patent Document 2: Japanese Patent Application Laid-Open No. 2004-328474
Patent Document 3: Japanese Patent No. 3755149
Patent Document 4: Japanese Patent Application Laid-Open No. 2005-067999